T-Mobile is trying to cheat me out of 800.00 Bellevue Washington

I returned an IPhone 8 to T-Mobile with their return label within the proper time by UPS. It was received at their return facility and signed for by a T-Mobile employee. When I noticed that they were still billing me for the phone I have called around 15 times and spoke to numerous supervisors who told me that they cannot find the phone. Even though I paid my bills for 3 years early and was told numerous times that I certainly do not fit the profile of someone that would do something wrong, they did tell me that they occasionally receive an empty box. The imei has never been active from on week before I returned the device untill now so it is nothing more than a paper weight. One rude employee said perhaps I sold it to China where he said they could somehow use it. I am so frustrated with them and find it hard to believe as someone who has 3 companies that you cannot talk to legal you must send them a letter. They wear you down little by little and I hope someone files a class action against this company.